The present invention concerns a device for evaluating the throughput of virtual circuits on an asynchronous time-division multiplex transmission channel.
An asynchronous time-division multiplex transmission channel carries data messages in digital data structures called cells. Each cell has a header comprising, for example, four 8-bit characters and a message body comprising a specified number of characters (32, for example). A continuous stream of such cells is carried by the transmission channel. If there is no message to be transmitted the transmission channel carries an "empty" cell, that is to say a cell with the same format as a message cell and containing conventional, easily recognizable information. Steps are taken to maintain a sufficient proportion of such empty cells in the stream of message cells; they are used, in particular, to synchronize the receiving end to the cell format.
The header of each message cell contains (on two characters, for example) information defining for the receiving end the direction in which the message body must be forwarded. The other two characters of the header contain service information and, in particular, code control and error detection information relating to the two destination characters. The same information is contained in the headers of irregularly spaced cells which have the same destination. It therefore identifies, so to speak, a virtual circuit occupying part of the transmission capacity of the transmission channel. More generally, this virtual circuit contributes a certain throughput on the transmission channel, measured in cells per unit time, for example, and this throughput fluctuates. A specific object of the invention is to evaluate the throughput as accurately as possible.
The transmission channel supports at any time a multiplicity of virtual circuits whose cells are interleaved in an irregular way by what is usually called asynchronous time-division multiplexing. The different virtual circuits have different, fluctuating throughputs. The sum of these throughputs is limited by the maximum throughput of the transmission channel, and also fluctuates. This leaves room for transmitting empty cells.
Also, the number of virtual circuits that can be separately identified depends on the number of bits assigned to this information in the cell header. The maximum number of virtual circuits is determined, among other things, by the number of virtual circuits obtained by dividing the maximum throughput of the transmission channel by the minimum throughput of a data source that can utilize a virtual circuit. This number is very large, for example 64 K.
However, asynchronous time-division multiplex transmission is used in a very wide field of applications and the throughputs of sources that can utilize a virtual circuit cover a vast range of throughputs (for example, from a few kilobits to several hundreds of megabits per second). The number of active virtual circuits will therefore usually be much less than the maximal number.
The above definition of asynchronous time-division multiplex transmission must not be limited to the case where all the cells are the same length. It is feasible to use cells of different lengths which are all multiples of a base length and in the context of the present invention the necessary adaptations will be evident to those skilled in the art.
An asynchronous time-division multiplex transmission channel is therefore designed to carry data supplied by sources with extremely varied and fluctuating throughputs. The forward switching and transmission equipments route the messages contained in the cells to their destination. To prevent the risk of forward congestion, it is necessary to prevent any source deliberately or accidentally contributing a throughput greater than the overall throughput assigned to it, even temporarily.
One known solution to this problem is known as "repression". The transmission channel is prevented from routing any cell regarded as being in excess of the overall throughput assigned to the virtual circuit, or at the very least the excess cell is marked as such so that it can be rejected further along the link if congestion occurs.
The application of a solution of this kind requires measurement of the throughput on the virtual circuit, this measurement possibly leading to the marking of a supernumerary cell.
The document FR-A-2 616 024 discloses a throughput measuring device which comprises a counter incremented by one step on receiving a cell of the virtual circuit to which it is assigned and decremented by one step on each clock pulse. If the virtual circuit throughput is low the counter is decremented more frequently than it is incremented and eventually locks in a minimal position; on the other hand, if the frequency of the cells of the virtual circuit is higher in the long term than the frequency of the clock pulses, the counter eventually locks into a maximal position and can then cause the marking of the cell as a supernumerary cell as mentioned above.
A system of this kind has the disadvantage that it can be applied only to a limited number of virtual circuits by virtue of the time needed to decrement the counters. It is not applicable to the previously envisaged situation in which the number of virtual circuits is very large.